You don't even need a pool table, just a table or counter.
Step 1: Check the shaft
Take the shaft and put it on a table or counter with some light behind it. Slowly turn the shaft and note any difference in the gap - you will immediately see if there is any warpage whatsoever.
Step 2: Check the handle
Do the same with the handle section pressing down on the butt. Note any change in the gap and/or lifting or falling of the joint.
Step 3: Check the joint alignment
Screw the cue together, gunsight down the cue and slowly turn it. If both parts are straight but the joint is out of alignment you will immediately see it because the entire shaft will be turning at an angle to the handle.
The reason why I use the above system is because it not only tells you if the cue is straight, it tells you what it is warped or out of alignment.
Chris